Method of integrating inorganic light emitting diode with oxide thin film transistor for display applications

ABSTRACT

A method of fabricating an active matrix display is disclosed in which one or more oxide thin film transistors is monolithically integrated with an inorganic light emitting diode structure. The method comprises forming an array of inorganic light emitting diodes over a substrate defining a plurality of sub-pixels, depositing an insulating layer over the inorganic LED array, forming conductive vias through the insulating layer, one via for each LED in the LED array, and forming a metal oxide thin film transistor backplane, including an array of pixel driver circuits, over the dielectric layer and conductive vias, wherein one driver circuit electrically controls each sub-pixel through the dielectric layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to provisional patent application Ser.No. 62/139,888 filed in the United States Patent and Trademark Office onMar. 30, 2015, the entire disclosure and drawings of which areincorporated in their entirety by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to methods of integrating lightemitting diodes (LEDs) with thin film transistors (TFTs) for displayapplications, and more particularly to a method of monolithicallyintegrating inorganic LEDs with metal oxide TFTs.

BACKGROUND OF THE INVENTION

Two kinds of light emitting diodes (LED) exist today. One is based onorganic materials (OLED), while the other one is based on inorganicmaterials. Both kinds of LEDs essentially work on similar principleswherein positive and negative charge carriers are injected into asemiconducting material and light emission occurs when the chargecarriers recombine in the light emission zone of the device stack. Bothmethods have their own advantages and disadvantages. OLEDs areattractive because such devices need not be produced on a crystallinesubstrate, the cost of producing such devices is low, the devices arepower efficient operating at low voltages, the devices have flexiblepotential, and the organic material enable devices that emit light in avariety of attractive colors. However, OLEDs devices lack very highbrightness and are limited in lifetime. Inorganic LEDs, on the otherhand, are attractive because they enable extremely robust devices, whichcan exhibit very high efficiencies. In addition, the lifetime ofinorganic LED devices is extremely long. The notable drawbacks ofinorganic LEDs include limitations in fabrication technology and thehigh cost of fabrication.

Until now, the main problem with fabricating inorganic LED technologyfor display applications was the drive circuit. Inorganic LEDs aregenerally made with compound semiconductor material including groupIII-V or II-VI materials, for example, Gallium Nitride (GaN). Compoundsemiconductor materials require very high temperature for processing(>700 C). The drivers made using compound semiconductor technologyexhibit significantly higher voltages compared to the standard silicontechnology such as complimentary metal-oxide semiconductor (CMOS) withsingle crystal Su, TFTs using a-Si or poly-Si. Therefore, a displaydevice using just GaN had not been feasible. There have been manyattempts to combine the Si technology for driving the GaN LEDs tofabricate displays, however, they have been largely unsuccessful. Assuch, inorganic LEDs are still not a part of the display industry.

The display industry continues to rapidly change with research anddevelopments improvements which reduce power consumption, improve imageresolution, and decrease device thickness. While at the same time, theemersion of new trending markets including wearable electronics withflexible and bendable displays, has begun to reshape technologyrequirements at many levels including the backplane level.

Generally, pixels in a flat panel display are arranged in a matrix form,and generate light (luminescence) upon electrical activation from anarray of thin-film-transistors, also known as TFT backplane. A TFTbackplane is an important part of display applications as it functionsas a series of switches to control the current flowing to eachindividual pixel. Until recently, there have been two primary types ofTFT backplane technologies, one using TFTs with amorphous silicon (a-Si)active layer and the other using TFTs with polycrystalline silicon(poly-Si) active layer. Currently, backplane technologies rely on thepresence of conventional silicon materials. The present inventionteaches methods of integrating TFTs for display applications usingnonconventional materials.

Therefore, there exists a need to provide a feasible and economicallyviable method of fabricating display devices integrating inorganic LEDswith metal oxide semiconductors (metal oxide TFTs), such as zinc oxide(ZnO) and indium gallium zinc oxide (IGZO).

It is therefore a primary object of the present invention to provide anew and improved method for monolithically integrating inorganic LEDswith metal oxide TFTs using nonconventional materials with hightransparency in the visible spectrum and controllable carrierconcentration.

It is another object of the present invention to provide a new andimproved method for fabricating flexible and transparent devices atlower temperatures.

It is another object of the present invention to provide an activematrix display incorporating an oxide TFT directly with a III-V (e.g.GaN) LED array.

It is another object of the present invention to provide an activematrix display with an extremely high brightness.

It is another object of the present invention to provide an activematrix display having higher contrast.

It is another object of the present invention to provide an activematrix display having longer lifetime.

SUMMARY OF THE INVENTION

The present invention cures the deficiencies in the prior art byproviding a method for integrating inorganic LED with oxide TFT fordisplay applications. An oxide TFT is a kind of field-effect transistormade by depositing thin forms of semiconductor active layer as well asthe dielectric layer and metallic contacts over a supporting substrate.In oxide TFT the material of the electron channel is oxide. The presentmethod involves the deposition of oxide TFT layers onto a previouslyfabricated GaN type LED substrate. By incorporating an oxide TFT with aIII-V type (e.g. GaN) LED array an active matrix display is realizedwith extremely high brightness, high contrast, and extended lifetime.

In an illustrative embodiment of the present invention, a method offabricating an active matrix display is disclosed. The steps includeforming an array of inorganic light emitting diodes (LEDs) over asubstrate defining a plurality of sub-pixels, depositing an insulatinglayer over the inorganic LED array, forming conductive vias through theinsulating layer, one via for each LED in the LED array, and forming ametal oxide thin film transistor backplane, including an array of pixeldriver circuits, over the dielectric layer and conductive vias, whereinone driver circuit electrically controls each sub-pixel through thedielectric layer.

The inorganic LED array may include forming a first type semiconductivelayer on the substrate, and a second or opposite type semiconductivelayer formed on the first type semiconductive layer. The emission layermay be positioned between the first and second semiconductive layers.The method may further comprise an n-electrode connected to the firsttype semiconductive layer and a p-type electrode connected to the secondtype semiconductive layer. The first and second semiconductive layersmay comprise III-V or II-VI group semiconductor materials. The III-Vcompound semiconductor may be GaN. The first type semiconductive layermay be an n-type GaN semiconductor. The n-electrode may be connected tothe n-type GaN semiconductor. The second type semiconductive layer maybe a p-type GaN semiconductor. The p-electrode may be connected to thep-type GaN semiconductor.

The metal oxide thin film transistor backplane may be formed over theinsulating layer by forming a gate electrode made of a conductivematerial formed over a portion of the insulating layer, depositing agate insulating film on the gate electrode, depositing a metal oxidesemiconductor layer, patterning the semiconductor layer to form achannel region on the gate insulating film, and depositing a sourceelectrode and drain electrode over the gate insulating film.

The display may further include a first conductive via plug contactingthe n-electrode of the LED to the source electrode of the thin filmtransistor, and a second conductive via plug contacting the p-electrodeof the LED to the drain electrode of the thin film transistor. Thesubstrate may be transparent or sapphire. The insulating layer may besilicon dioxide.

In accordance with another illustrative embodiment of the presentinvention, a method of fabricating a light emitting device is disclosed.The steps include forming a compound stacked semiconductor structureover a substrate, wherein the semiconductor structure includes an n-typetype semiconductive layer formed on the substrate comprising a materialselected from the group consisting of III-V and II-VI compounds, ap-type semiconductive layer overlying the n-type semiconductive layercomprising a material selected from the group consisting of III-V andIV-VI compounds, electrically coupling a first electrode with the n-typesemiconductive layer, and electrically coupling a second electrode withthe p-type semiconductive layer. Then, forming an insulating layercomprising dielectric material over the semiconductor structure in amanner which does not cause significant damage to the structure. Then,forming conductive vias through the insulating layer, and forming ametal oxide thin film transistor backplane over the insulating layer andconductive vias in a manner which does not cause significant damage tothe semiconductor structure.

The method may include forming a metal oxide thin film transistorbackplane over the insulating layer by forming a gate electrode made ofa conductive material formed over a portion of the insulating layer,depositing a gate insulating film on the gate electrode, depositing ametal oxide semiconductor layer, patterning the semiconductor layer toform a channel region on the gate insulating film, and depositing asource electrode and drain electrode over the gate insulating film.

The method may include a first conductive via contacting the firstelectrode of the semiconductor structure to the source electrode of thethin film transistor, and a second conductive via contacting the secondelectrode of the semiconductor structure to the drain electrode of thethin film transistor.

The conductive vias may be formed by patterning by etching.

In another illustrative embodiment of the present invention, a method offabricating a light emitting device is disclosed by integrating acompound stacked semiconductor structure over a substrate, wherein thesemiconductor structure includes material selected from the groupconsisting of III-V and II-VI compounds; forming an insulating layerover the semiconductor structure; and forming a metal oxide thin filmtransistor backplane over the insulating layer.

In another illustrative embodiment of the present invention, a devicecomprising an array of inorganic LEDs made by the method described aboveis disclosed. The device may be a display or a microdisplay.

These advantages of the present invention will be apparent from thefollowing disclosure and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

To these and to such other objects that may hereinafter appear, thepresent invention relates to methods of monolithically integratinginorganic LEDs with metal oxide TFTs for display applications asdescribed in detail in the following specification and recited in theannexed claims, taken together with the accompanying drawings, in whichlike numerals refer to like parts in which:

FIG. 1 is a layer diagram illustrating a GaN LED structure in a processfor integrating light emitting diode with oxide thin film transistoraccording to an illustrative embodiment of the present invention.

FIG. 2 is a layer diagram of the GaN LED of FIG. 1, illustrating thesteps in the process of forming an insulation layer over the GaN LEDaccording to an illustrative embodiment of the present invention.

FIG. 3 is a layer diagram illustrating the steps in the process offabricating a gate dielectric layer and vias for connecting electrodesof the LED to driver circuits above the insulation layer according to anillustrative embodiment of the present invention.

FIG. 4 is a layer diagram illustrating the steps in the process offabricating a TFT array above the LED array of FIG. 4 according to anillustrative embodiment of the present invention.

FIG. 5 is a layer diagram illustrating an oxide TFT structure in aprocess for integrating light emitting diode with oxide thin filmtransistor according to an illustrative embodiment of the presentinvention.

DETAILED DESCRIPTION

The present invention is directed to methods of integrating lightemitting diodes (LEDs) with thin film transistors (TFTs) for displayapplications. In particular, the present invention utilizes III-V orII-VI group compound semiconductor materials to form LED devices. Thesematerials require temperatures greater than 700 C for processing. Itshould be noted that the materials and processes described herein arefor illustrative purposes and the present invention is not limited tothe specific LED devices or TFT configurations described.

FIGS. 1-4 illustrate the sequence of steps used in fabricating a TFTarray on a LED array, having a plurality of sub-pixels, according to thepresent method. FIG. 1 illustrates an LED wafer 10 according to anillustrative embodiment of the present invention. The LED wafer 10includes a stacked structure 14 composed on a transparent substrate 12.The structure 14 includes a first type semiconductive layer 16positioned on the substrate 12, a p-n junction 18 containing a lightemission region, a second type semiconductive layer 20 on top of the p-njunction 18, a p-type electrode 22, and an n-type electrode 24. The LEDwafer 10 is patterned into an array of LEDs. It should be understoodthat the LED wafer 10 may be formed in accordance with techniques knownin the art, such as by forming, for example, a gallium nitride (GaN) LEDstructure on a base sapphire substrate. It should be noted that whilethe term “wafer” is used to depict the overall structure, any wafer,portion of a wafer, chip, etc. may be used.

In the preferred embodiment, the transparent substrate 14 is sapphire.The first type semiconductive layer 16 is an n-type GaN semiconductor,which is formed over the sapphire substrate 12. The p-n junction 18, oremission layer, is then formed over the n-type GaN semiconductor. Thesecond type semiconductive layer 20 is a p-type GaN semiconductor, whichis formed over the emission layer 18. Since sapphire is dielectric, aportion of the stacked structure 14 of the LED wafer 10 has to beetched, exposing a portion 26 of the n-type GaN semiconductor layer 16.The n-type electrode 24 is formed on the exposed portion 26 of then-type GaN semiconductor layer 16. The p-type electrode 22 is formed onthe p-type GaN semiconductor layer 20. It should be noted that the firstsemiconductive layer may be either n or p type conductivity, and thesecond semiconductive layer is of opposite conductivity material (i.e. por n).

FIG. 2 illustrates the fabricated LED wafer 10 covered with aninsulating layer 30 of dielectric material, preferably silicon dioxide(SiO2). The insulating layer 30 is deposited over the array of LEDs bytechniques known in the art, which may include at approximately 1 umusing thermal oxidation techniques. The insulating layer 30 may have athickness in the range of 0.5 to 2 microns. One driver circuitelectrically controls each sub-pixel through the insulating layer 30.

FIG. 3 illustrates the fabrication of conductive via plugs 40 formedthrough the insulating layer 30. According to an illustrative embodimentof the present invention, patterning the insulated layer 30, which mayinclude conventional photolithographic techniques to mask portions ofthe insulating layer 30 and to expose portions where future vias 42 aare to be formed, forms the conductive vias 42 a. The exposed portionsare then removed by etching or other suitable removal technique known inthe art. One via is formed for each LED in the LED array. The method ofpatterning silicon dioxide is well known to experts in the field.

Following the patterning of the insulated layer 30, conductive via plugs40 are formed within vias 42. According to an illustrative embodiment ofthe present invention, a material deposition system is used to deposit aconductive material, for example (ADD), in vias 42 for forming via plugs40 one at a time. It should be noted that other systems and methodsknown in the art may be similarly used.

FIG. 4 illustrates a sectional view of the steps forming layers of themetal oxide thin film transistor structure 10 over the insulating layer30 in accordance with an illustrative embodiment of the presentinvention. The TFT structure 50 has source 58, drain 60, channel 56,gate insulator 54, and gate 52 regions.

The gate metal 52 is deposited and patterned onto the insulating layer,preferably using a mask technique. It should be noted that other methodsknown in the art may be similarly used. The gate insulator layer 54 ofdielectric material, is then deposited over the gate metal 52.

Vias are needed to connect the gate metal to subsequent metal layers tocomplete various circuits in the active matrix. In other embodiments,vias between the gate metal layer and the source/drain metal layer canbe made easily and efficiently by using a laser. In other embodiments,vias are formed using photoresist, patterning, and an anisotropicetching. Vias 42 b are opened through the gate insulating layer 54. Ametal oxide semiconductor layer is then deposited as a blanket layer andpatterned to form the channel 56 for a TFT.

The blanket source and drain metallization layer is then deposited overthe structure and patterned to separate the metal layer into source anddrain electrode contacts 58, 60. In other embodiments, laser dopingtechniques known in the art, may be used to form the TFT source anddrain regions. The source and drain contacts, 58 and 60 respectively,are on opposite sides of the active area of the channel 56 and form acontact with the n-electrode 24 and p-electrode 22 through vias 42.

It is to be understood that the disclosure describes a few embodimentsand that many variations of the invention can easily be devised by thoseskilled in the art after reading this disclosure and that the scope ofthe present invention is to be determined by the following claims.

What is claimed is:
 1. A method of fabricating an active matrix display,the steps comprising: forming an array of inorganic light emittingdiodes (LEDs) over a substrate defining a plurality of sub-pixels,wherein the inorganic LED array including a first type semiconductivelayer on the substrate and a second or opposite type semiconductivelayer formed on the first type semiconductive layer, further includingan n-electrode connected to the first type semiconductive layer and ap-type electrode connected to the second type semiconductive layer;depositing an insulating layer over the inorganic LED array; formingconductive vias through the insulating layer, one via for each LED inthe LED array including a first conductive via plug and a secondconductive via plug; forming a metal oxide thin film transistorbackplane, including an array of pixel driver circuits, over thedielectric layer and conductive vias, wherein one driver circuitelectrically controls each sub-pixel through the dielectric layer, andfurther including: forming a gate electrode made of a conductivematerial formed over a portion of the insulating layer; depositing agate insulating film on the gate electrode; depositing a metal oxidesemiconductor layer; patterning the semiconductor layer to form achannel region on the gate insulating film; depositing a sourceelectrode and drain electrode over the gate insulating film; and whereinthe first conductive via plug contacting the n-electrode of the LED tothe source electrode of the thin film transistor, and the secondconductive via plug contacting the p-electrode of the LED to the drainelectrode of the thin film transistor.
 2. The method of claim 1 whereinan emission layer is positioned between the first and secondsemiconductive layers.
 3. The method of claim 2 wherein the first andsecond semiconductive layers are comprised of III-V or II-VI groupsemiconductor materials.
 4. The method of claim 3 wherein the III-Vcompound semiconductor is GaN.
 5. The method of claim 4 wherein thefirst type semiconductive layer is an n-type GaN semiconductor.
 6. Themethod of claim 5 further comprising an n-electrode connected to then-type GaN semiconductor.
 7. The method of claim 4 wherein the secondtype semiconductive layer is a p-type GaN semiconductor.
 8. The methodof claim 7 further comprising a p-electrode connected to the p-type GaNsemiconductor.
 9. The method of claim 1 wherein the substrate istransparent.
 10. The method of claim 1 wherein the substrate issapphire.
 11. The method of claim 1 wherein the insulating layer issilicon dioxide.
 12. A method of fabricating a light emitting device,comprising the steps of: forming a compound stacked semiconductorstructure over a substrate, wherein the semiconductor structureincluding: an n-type type semiconductive layer formed on the substratecomprising a material selected from the group consisting of III-V andII-VI compounds; a p-type semiconductive layer overlying the n-typesemiconductive layer comprising a material selected from the groupconsisting of III-V and IV-VI compounds; electrically coupling a firstelectrode with the n-type semiconductive layer; and electricallycoupling a second electrode with the p-type semiconductive layer;forming an insulating layer comprising dielectric material over thesemiconductor structure in a manner which does not cause significantdamage to the structure; forming conductive vias through the insulatinglayer; forming a metal oxide thin film transistor backplane over theinsulating layer and conductive vias in a manner which does not causesignificant damage to the semiconductor structure including: forming agate electrode made of a conductive material formed over a portion ofthe insulating layer; depositing a gate insulating film on the gateelectrode; depositing a metal oxide semiconductor layer; patterning thesemiconductor layer to form a channel region on the gate insulatingfilm; and depositing a source electrode and drain electrode over thegate insulating film; and providing a first conductive via contactingthe first electrode of the semiconductor structure to the sourceelectrode of the thin film transistor; and a second conductive viacontacting the second electrode of the semiconductor structure to thedrain electrode of the thin film transistor.
 13. The method of claim 12wherein the conductive vias are formed by patterning by etching.
 14. Adevice comprising a compound stacked semiconductor structure over asubstrate with a metal oxide thin film transistor backplane made by themethod of claim
 12. 15. The device of claim 14, wherein the device is adisplay.
 16. The device of claim 14, wherein the display is amicrodisplay.